Compact and thin electronic apparatuses include a large number of flexible printed wiring boards. To achieve higher-density and thinner electronic apparatuses, the flexible printed wiring boards having a multilayer structure have been more required. In addition, the flexible printed wiring boards are required to be of a higher quality. As materials for realizing multilayer structures of flexible printed wiring boards, bonding sheets or coverlays are adopted as insulating layers, and metal foils such as copper foils are adopted as conductive layers. To satisfy the demand for a reduction in thickness, a resin-clad metal foil has been proposed (e.g., see Patent Literature 1).
Moreover, in a known method for fabricating a multilayer printed board (e.g., see Patent Literature 2), a perforated substrate having holes, a film-shaped adhesive having holes corresponding to the holes in the substrate, and a printed board having a copper pattern are hot pressed with the film-shaped adhesive being sandwiched between the perforated substrate and the printed board to mold a stack. In this method, in order to prevent the film-shaped adhesive from being extruded into the holes in the perforated substrate, the hot pressing is performed with a thermoplastic resin sheet being disposed on the perforated substrate to mold the stack.
In order to reduce the thickness of the multilayer printed board, a perforated resin-clad metal foil may be used instead of the perforated substrate and the adhesive film. Specifically, the printed board, the perforated resin-clad metal foil, and a thermoplastic resin sheet are hot pressed with a resin part of the perforated resin-clad metal foil being disposed on the printed board and the thermoplastic resin sheet being disposed on a metal foil part of the perforated resin-clad metal foil to mold a stack. During the molding, the thermoplastic resin sheet is thermally deformed to fill the holes in the perforated resin-clad metal foil, which enables extrusion of a resin of the perforated resin-clad metal foil into the holes to be reduced.
The printed board has, however, a surface which is provided with the copper pattern and is thus uneven. Thus, when the hot pressing is performed with the thermoplastic resin sheet being provided, the unevenness of the surface of the printed board may result in an uneven surface of the metal foil of the perforated resin-clad metal foil. When the surface of the metal foil is uneven and is no longer smooth, mounting components on the surface with a high density becomes difficult.
Thus, adopting a resin having a high degree of fluidity as a resin of the perforated resin-clad metal foil without providing the thermoplastic resin sheet is considered. In this case, the resin is more likely to flow during the molding and the unevenness of the surface of the printed board is smoothed to secure the smoothness of the surface of the metal foil on the printed board. Moreover, the adhesive property, bending property, thermal resistance, and circuit filling property of the printed board are good. However, since the resin of the perforated resin-clad metal foil has a high degree of fluidity, the resin is extruded into the holes.